1. Field of the Invention
This invention relates to the field of gas separation, and more particularly to gas separation using a hydroxide ion conductive membrane.
2. Description of the Prior Art
There is an ever-increasing need for improved systems and methods for efficient and rapid separation of selective components from mixtures. In particular, techniques for the separation of selective components from gaseous mixtures have many significant technical applications. Many important chemical, environmental, medical and electronics processing technologies require pure gas, particularly pure oxygen gas. For example, oxygen is used in semiconductor fabrication for chemical vapor deposition, reactive sputtering and reactive ion etching. It finds wide application in health services, for resuscitation, or, in combination with other chemicals, for anesthesia. Oxygen can also be used to achieve environmental benefits by reducing the sulfur emissions of oil refineries and helping pulp and paper manufacturers meet regulations relating to bleaching, delignification and lime kiln enrichment. However, the high cost of pure oxygen typically limits the wide adoption of such beneficial processes in the chemical, electronics and medical industries. Further, nitrogen is used, for example, to protect perishable goods, to protect oxygen-sensitive materials and to facilitate oxygen sensitive processes.
High-purity oxygen (e.g., 99.5%+) is largely produced in cryogenic air separation plants, where the air is cooled down to the melting point of nitrogen (xe2x88x92210xc2x0 C.) and its components separated in large condensation columns. This process requires expensive, bulky equipment and high-energy consumption, which tends to militate against the use of oxygen to generate energy. Cryogenic air separation is typically commercially feasible only on large scale generation systems.
Adsorption, both pressure-swing absorption (PSA) and vacuum-swing absorption (VSA), is widely used for producing moderate purity oxygen (e.g., 85-90%). The gas separation is generally based on selective adsorption of nitrogen by synthetic zeolites. Nitrogen, which is more readily polarizable than oxygen, interacts more strongly with the electrostatic fields in the zeolites structure. Nitrogen is thus retained by the zeolites, while oxygen passes through. When the zeolite is saturated with nitrogen, it must be stripped, generally by reducing the pressure.
Polymer membrane air separation has also been used to produce air slightly enriched in oxygen (e.g., 28-35%) or for nitrogen enriched blanketing (e.g., 95-99%). Polymeric air separation membranes are typically selectively permeable, whereby the polymeric materials are more permeable to oxygen than nitrogen. Transport through the membrane is induced by maintaining the vapor pressure on the permeate side of the membrane lower than the vapor pressure of the feed mixture. The driving force is typically the difference in partial vapor pressure of each species across the membrane.
One typical configuration includes a system of hollow fibers, wherein air is passed over the exterior of the fibers, and enriched oxygen permeates to the interior of the fibers. Another typical configuration used a spiral wound system, wherein air is passed through an edge of a configuration of feed channels, membranes and a permeate channel wound about a porous tube. The permeate exits the system from the porous tube. Production of relatively high purity oxygen with air separation based on selective permeability is not commercially feasible, and is only suitable for applications where only oxygen-enriched air is required.
The permeability of a membrane for a particular gas species is related to the volumetric flow (cubic centimeters (at standard pressure and temperature) per second (cc(STP)/s)) of the desired species through a unit length of membrane thickness (in centimeters) per area of membrane (in centimeters squared) per pressure differential (expressed in centimeters of mercury). The permeability is generally expressed in Barrer units, where:                               1          ⁢                      xe2x80x83                    ⁢          Barrer                =                  1          xc3x97                                                    10                                  -                  10                                            ⁡                              [                                                                            cc                      ⁡                                              (                        STP                        )                                                              ·                    cm                                                                                                      cm                        2                                            ·                      s                      ·                      cm                                        ⁢                                          xe2x80x83                                        ⁢                    Hg                                                  ]                                      .                                              (        1        )            
Additionally, selectivity is critical to effective permeability based gas separation, such that a desired species permeates at a greater rate than another species. Selectivity (alpha) is generally expressed as:                               α                      A            ,            B                          =                                            Permability              A                                      Permability              B                                .                                    (        2        )            
A relatively new technology that has emerged for relatively high purity gas separation involves selective membranes which pass only the desired components, such as described by H. J. M. Bouwmeester, A. J. Burggraaf, in xe2x80x9cThe CRC Handbook of Solid State Electrochemistry,xe2x80x9d Ed. P. J. Gellings, H. J. M. Bouwmeester, chapter 11, CRC Press, Boca Raton, 1997, which is incorporated by reference herein. Various known membranes include ionic conducting membranes and mixed ionic-electronic conducting membranes, which rely on the transportation of oxide (O2xe2x88x92) ions to separate the oxygen from air. Although these approaches may offer some advantages relative to cryogenic oxygen separation, practical application of the MIEC membrane is hindered by a number of drawbacks intrinsic to oxide (O2xe2x88x92) conductive membranes. These problems include: low oxygen throughput (typically caused by both low ionic conductivity and low surface oxygen exchange rate); relatively high operating temperature ( greater than 800xc2x0 C.); costly materials and costly fabrication; tendency to degrade over time; and system equipment that is relatively complex and expensive to build and maintain.
Turning now to FIG. 1a, a general process that occurs in a conventional ionic-electronic membrane 10 is shown. Air arrives at a cathode 12, where oxygen is reduced, but other species do not react. The oxygen is shuttled across a membrane 10 in the form of an ion such as O2xe2x88x92 (the process of ionic conduction). At an anode 14, a complementary chemical reaction evolves pure oxygen, which is released. Functionally, this type of membrane 10 has three primary components: a backbone 16, which provides the membrane""s structure; an ionic conductor (not shown), which conducts the ions across the membrane; and a catalyst (not shown), which aids the reduction of oxygen at the cathode 12 and the evolution of oxygen at the anode 14.
The overall oxygen throughput is determined primarily by two parameters (the conductivity of the electrons is generally too fast to be a limiting factor). The first is the ionic conductivity (how fast the ions can travel across the membrane), which is dependent on the electrolyte properties. The second is the surface oxygen exchange rate (how quickly the oxygen is reduced and evolved on each side), which is dependent on both the ionic conductor and the catalyst properties of the electrodes.
Referring now to FIG. 1b, prior art mixed ionic-electronic conducting membranes 110 typically utilize only a single component, i.e., a ceramic-noble metal composite, to play all three functional roles, namely, backbone, ion conduction and catalyst. This material provides the physical membrane structure 116, reduces and evolves the oxygen, and relays the O2xe2x88x92 ions and charge-compensating electrons in opposite directions as shown. To drive the reaction, a pressure differential is required, with the air pressure at the cathode 112 exceeding the oxygen pressure at the anode 114, i.e., by a factor of about 2 to 10.
As shown in FIG. 1c, a similar approach known as xe2x80x9cactive oxygen pumpingxe2x80x9d utilizes a membrane 110 in combination with an external circuit 118, instead of a pressure differential, to drive the reaction.
As mentioned hereinabove, there are several disadvantages associated with the current ceramic-based membranes (ionic conducting and mixed ionic-electronic conducting) approach, which are intrinsic to oxide-conducting membranes.
One such disadvantage is their relatively high operating temperatures. The chemistry of the ceramic-noble metal membrane material requires temperatures near 800xc2x0 C. for the anode and cathode reactions, as well as the ionic conduction, to proceed.
A further disadvantage of ceramic membranes is their relatively high material and production cost. Since ceramic membranes generally use the same material to conduct the oxide ions and as the backbone material, restrictions are placed on the possible materials that may be used. In particular, noble metals such as platinum and palladium are generally required to obtain desirable stability at required operating temperatures and to promote the oxygen surface reaction, and these metals are relatively expensive. Also, fabrication of the backbone structure requires relatively strict and careful control to produce the correct density and degree of mixing between the ceramic and the metal. This tends to increase the expense of the process and lower overall yield.
Another disadvantage of ceramic based membranes is relatively low long-term stability under their operating conditions. At the 800xc2x0 C. operating temperature of these membranes, the ceramic and the noble metal tend to react with one another, generating oxidization of the metal and a concomitant degradation in performance through lower conductivity. This instability generally renders such a system impractical for large-scale applications.
An engineering problem associated with systems that incorporate these high-temperature, high-pressure oxide-conducting membranes involves sealing the membrane so that air cannot leak past it. Any such leakage tends to disadvantageously lower the purity of oxygen on the downstream side. This problem has been addressed by welding the sides of the membrane to input and output gas lines, but it is a relatively costly and troublesome solution.
All of the above problems, which are intrinsic to the ceramic-noble metal system, illustrate the undesirability of current oxide-conducting membrane technology.
Additional ionic conducting membranes have been proposed based on cation (i.e., proton) exchange membranes, which typically utilize fluorocarbon-type resins (e.g., the Nafion(copyright) family of resins which have sulfonic acid group functionality, commercially available from DuPont Chemicals, Wilmington, Del.), precious metal (e.g., Pt) anodes and air cathodes. In such systems, separation is based on proton (i.e., hydrogen cation) conduction through the membrane. Oxygen reduces at the cathode and forms water with a provided electrical voltage and hydrogen cations in the membrane, and evolves at the anode from water. Commercial realization of cation exchange membrane based air separation systems is limited by the costs of both the membrane and the catalysts materials.
Thus, a need exists for an oxygen separation method and apparatus that addresses the problems associated with conventional air separation techniques and systems.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the several methods and apparatus of the present invention, wherein a hydroxide-conducting material is utilized to separate gases. An important aspect is that hydroxide ions (OHxe2x88x92), rather than oxide ions (O2xe2x88x92) or protons, may be utilized to shuttle oxygen molecules through a membrane at relatively high oxygen throughput. The hydroxide ion generally has higher conductivity than the oxide ion at any given temperature. Also, the surface oxygen exchange rate is higher in an alkaline electrolyte than in oxide electrolytes and acidic electrolytes (such as in the proton exchange membrane), especially at low temperature.
The present invention provides, an electrochemical cell for separating a first gas from a mixture of gas is provided, particularly for separating oxygen from air. The cell includes a first electrode, a second electrode and a hydroxide-conducting membrane between the first electrode and the second electrode.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.